The present invention relates to a polyethylene-based composition, to a process for the manufacture of shaped articles and to the articles shaped from this composition. It relates more particularly to a composition based on high density polyethylene containing small amounts of finely divided inorganic additive and to the articles shaped from this composition, in particular pipes intended for the transportation of fluids and in particular of pressurized fluids.
It is known to improve the antiblocking properties of films made of low to medium density polyethylene by incorporation, in the polyethylene, of small amounts (approximately 0.05 to 1%) of finely divided inorganic materials, such as talc, kaolin or silica (Italian Patent 719,725). More recently, the incorporation of 200 to 2500 ppm (0.02 to 0.25%) of talc has been specifically recommended in order to improve the antiblocking properties of films made of polyethylene having a density of between 0.905 and 0.935 g/cm3, without having a significant effect on the transparency (Patent EP-B-60178).
Furthermore, it is known to use polyethylene and in particular high density polyethylene for producing pipes and couplings for the transportation of pressurized fluids. In this application, it is essential, for obvious reasons of safety and of longevity of fluid transportation plants, to have available shaped articles, such as pipes and couplings, which exhibit very high resistance to hydrostatic pressure.
The aim of the present invention is to provide a composition based on high density polyethylene which exhibits a markedly improved resistance to hydrostatic pressure and consequently a markedly improved creep resistance.
To this end, the invention relates to a composition based on polyethylene, the standard density of which, measured at 23° C. according to ASTM Standard D 792, is greater than 940 kg/m3, comprising talc in an amount of less than 1 part per 100 parts by weight of polyethylene.
The talc which can be used in the compositions according to the invention can be any magnesium silicate hydrate of natural origin of general formula 3MgO.4SiO2.H2O. It can contain minor amounts of metal oxides, such as aluminium, iron and calcium oxides. The talc advantageously has an essentially lamellar texture. Its particle size distribution is preferably situated between 0.2 and 15 microns and its mean particle size between 1 and 5 microns.